The present invention relates to fluid flow controllers for fluid conduits and in particular to flow controllers adapted to control the flow of fluid through a conduit to a zone at a selectable constant volume value which can be varied in response to feedback from changes in the zone caused by the flow of fluid thereto.
In large environmental air conditioning systems, a problem exists in maintaining many zones within the environment at a desired temperature by means of heated or cooled air from a common source. Various pressure drops are caused within the conduits connecting the fluid (air) source to the many outlets within the environment.
Heretofore, the problem of maintaining a zone at a desired temperature by means of a controlled fluid flow into the zone has been solved in one of two basic manners. First, as shown in U.S. Pat. No. 3,227,369 for example, the proportions of hot and cold input fluid flowing into the zone can be thermostatically controlled. The aforementioned pressure drops within the system still cause variations in zone temperature with such an approach. Dampers can be added in the various conduits at points adjacent each zone to be controlled with a transducer (i.e. a thermostat) controlling each damper disposed within the zone to be controlled by the particular conduit. Additionally, the temperature mixing operation can be performed at a plurality of sites disposed throughout the environment to be controlled. All of the foregoing leads to an increasingly complex and costly system which, in turn, tends towards malfunction because of the complexity thereof.
The second approach is a more simplified approach well known in the art for many years. According to this technique, a single temperature fluid is provided in variable volumes to the zones to be controlled.
Until recently, the older approach of providing a single temperature fluid at variable fluid flow volumes was found objectionable as a non-constant volume flow rate of fluid supplied to a zone not only caused undesirable changes in the noise level of the device, but also prevented properly supplying the zone with fresh fluid in a uniform manner. In the recent U.S. Pat. No. 3,809,314, Engelke et al described a self-powered variable volume air damper control adapted "to regulate volume flow of air from a duct into a conditioned space solely as a function of a sensed condition regardless of air supply pressure". According to the teaching of Engelke et al, a tube is installed facing into the duct supplying the flowing fluid to the zone so as to develop a source of air pressure used to expand a bellows connected to a damper within the duct. An orifice plate is disposed in the duct with a differential pressure transducer connected across the orifice plate so as to respond to the static pressure drop across the orifice plate caused by the flow of fluid therethrough. The differential pressure transducer and a thermostat are each connected to control the bleed rate of air pressure out of the bellows whereby the flow through the duct of fluid is controlled as a combination function of the static pressure of the fluid, the flow rate of the fluid through the orifice plate, and the temperature within the zone. While tending towards solving the problems attendant the former non-constant volume flow rate installations of such air conditioning apparatus, the apparatus of Engelke et al presents shortcomings of its own. First, the differential pressure transducer does not measure fluid flow rate within the duct directly. That is, it estimates fluid flow rate as a function of the differential pressure drop across the orifice plate. As such, it is prone to inaccuracies due to positioning of the orifice plate, positioning of the openings into the duct supplying the differential pressure transducer, boundary layer effects of the fluid flow through the conduit, and eddy currents within the conduit, just to name a few. Additionally, such an approach provides two additional problems. First, it forces the use of a constant volume regulator (item 33 in the single figure). Second, it takes at least one inch of system pressure to operate the bellows controlling the damper. In light of the energy conservation programs presently in effect, systems operating at 10 percent of the minimum required pressure for operation of the Engelke et al apparatus (i.e. 1/10 inch of system pressure) are more desirable.
Wherefore, it is the object of the present invention to provide a fluid flow controller for fluid conduits able to maintain a variable value constant volume fluid flow through the conduit operable at low system pressures and responsive to true average volume flow across the diameter of the conduit through which the fluid flow is being controlled.